Gas turbines are widely used in commercial operations for power generation. A typical gas turbine includes a compressor at the front, one or more combustors around the middle, and a turbine at the rear. The compressor imparts kinetic energy to the working fluid (air) to bring it to a highly energized state. The compressed working fluid exits the compressor and flows to the combustors. The combustors mix fuel with the compressed working fluid, and the mixture of fuel and working fluid ignites to generate combustion gases having a high temperature, pressure, and velocity. The combustion gases flow to the turbine where they expand to produce work.
It is widely known that the thermodynamic efficiency of a gas turbine increases as the operating temperature, namely the combustion gas temperature, increases. Higher temperature combustion gases contain more energy and produce more work as the combustion gases expand in the turbine. However, higher temperature combustion gases may produce excessive temperatures in the turbine that can approach or exceed the melting temperature of various turbine components.
A variety of techniques exist to allow the combustors to operate at higher temperatures. For example, air may be extracted from the compressor, bypassed around the combustors, and injected directly into the stream of combustion gases in the turbine to provide conductive and/or convective cooling to the turbine stages. However, the air extracted from the compressor has already been compressed, and thus heated, by some amount, thereby reducing the heat removal capability of the extracted air. In addition, since the extracted air bypasses the combustors, extracting air from the compressor reduces the volume of combustion gases and overall efficiency and output of the gas turbine.
Another method to cool turbine components is to circulate air through the interior of the turbine components. For example, the turbine typically includes stationary nozzles or stators and rotating blades or buckets. The stators and/or buckets may include internal passages through which cooling air may flow. As the cooling air flows through the internal passages, the cooling air directly contacts the walls of the internal passages to remove heat from the stators and/or buckets through conductive or convective cooling. A disadvantage of this cooling method is the increased manufacturing costs associated with fabricating the detailed and contoured internal passages in the stators and/or buckets. In addition, the cooling air flowing through the internal passages preferably must be at a pressure greater than the combustion gases flowing outside of the turbine component to minimize the risk that the combustion gases may penetrate the stators and/or buckets, thereby eliminating any cooling provided by the cooling air. Lastly, removal of heat from the gas turbine, without producing work from that heat, necessarily reduces the overall thermodynamic efficiency of the gas turbine.
Therefore, the need exists for a cooling system that can remove heat from gas turbine components that avoids some or all of the disadvantages of existing systems. Ideally, the cooling system will provide cooling to the gas turbine components without increasing manufacturing costs or decreasing the overall operating efficiency of the gas turbine.